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DISCOVERING PLANETS IN THE RADIO SKY

While searching the radio sky, Alexander Wolszczan, professor of astronomy and astrophysics at Penn State University, discovered a new millisecond pulsar, PSR1257+12, with some exciting behavioral attributes. During the routine timing analysis associated with this discovery, Wolszczan recognized that PSR1257+12 seemed to “wobble” as it traveled through space. After further analysis, he was able to identify a pattern to the wobble. He proposed that there are two (perhaps three) planets orbiting PSR1257+12. Wolszczan believes that the planets’ gravitational pull is causing PSR1257+12’s unique behavior. When his findings are verified—and most of his peers expect that they will be —Wolszczan will go down in history as the first scientist to identify a planetary system beyond our own.

A pulsar is an extremely dense and small neutron star believed to have been born as a result of a supernova, or explosion of a large star from a class ten or more times the mass of the sun. PSR1257+12 (one of only twenty such stars identified in the galaxy) is about ten miles in diameter, has 1.4 times the mass of the sun, and emits beams of radio waves that have been focused by the star’s extremely strong, approximately dipolar magnetic fields. These signals are analogous to rotating lighthouse beams periodically sweeping across the universe.

Just as a sailor sees flashes of the beacon from a lighthouse, a radio telescope on earth receives pulsar emissions as intermittent but remarkably regular pulses. Scientists are able to detect pulsars by searching for faint periodic radio pulses in the blizzard of information coming from space. Only recently have computers and analytical techniques evolved that are capable of analyzing the millions of samples required to yield results that will stand up to scrutiny.

When searching for pulsars, astronomers examine data collected from the radio sky, or outer space as it appears to a radio, as opposed to an optical telescope. The astronomer’s ability to detect new pulsars is limited by the speed and accuracy of data-recording instruments and by the availability of computing resources. Detecting millisecond pulsars requires tremendous computing power. Samples for research like Wolszczan’s are recorded every 0.3 milliseconds (over 300,000 bits per second). Detecting pulsars is like looking for a subtle repeating color pattern among the blades of grass in a prairie as the breezes wax and wane with the cloud patterns—it’s a job for a supercomputer.

As Wolszczan analyzed the data collected over a 486-day period, he looked for a pattern in the arrival time of the pulses, searching millions of bits of information. Using the supercomputer facilities at the Cornell Theory Center, he determined that the pulses were arriving every 6.2 milliseconds. Wolszczan detected unusual complexity in the pattern of the pulses’ arrival times: they periodically arrived early and bunched together and then spread apart, as they began to arrive later than the predicted time. This behavior suggested that the pulsar’s motion is affected by the presence of other orbiting objects. Instead of moving steadily, PSR1257+12 was being pulled around a point in space called the barycenter—the center of mass of a system—by the gravitational interaction with these objects.

When Wolszczan tries to account for the forces causing the star to orbit, he comes up with a remarkable explanation. Wayne Lytle, visualization specialist at the Cornell Theory Center, has produced an animation of Wolszczan’s data that presents the pulsar orbiting around the barycenter of a system having two planets. These two planets are themselves orbiting every 66.6 days and 98.2 days respectively. If you allow for the gravitational pull of these two planets in the calculations, the rest of the problematic pattern is explained.